This application relates to and claims priority to corresponding German Patent Application No. 101 00 328.5 filed on Jan. 5, 2001.
The invention relates to a holding device for an optical element, while the optical element is being coated in a vacuum coating plant having a mounting-device for the optical element, it being possible for the optical element to be heated in the vacuum coating plant via suitable radiation.
In general, optical elements are very frequently coated with functional layers to improve the optical quality, for example antireflection coatings or the like. This coating is normally carried out in a vacuum coating plant, into which the optical elements are introduced and in which they are heated while, at the same time, the substances for the corresponding functional layer are fed in. The substances for the corresponding functional layer are then deposited on the surfaces of the optical elements. In order to achieve the most uniform deposition possible on the surfaces of the optical elements, the latter are inserted into corresponding mountings for holding the optical elements and are generally moved in the vacuum coating plant.
Since, then, lenses of this type or other optical elements made of crystalline materials, which are used for example in the semiconductor lithography technique, have relatively large diameters and are comparatively heavy, these devices for mounting the optical elements in the vacuum coating plant, with the corresponding driven axes, are designed from high-strength materials, in order for example to be able to coat four or more lenses simultaneously in the vacuum coating plant, without the mountings failing as a result of the weight loading together with the thermal loading which occurs. For this purpose, the holding elements are generally designed from a high-strength metal, for example steel.
Then, because of the vacuum or at least approximately complete vacuum, appropriate radiators must be used in order to ensure that the optical elements are heated up, since the transfer of heat as a result of convection or the like cannot be utilized in a vacuum. In the case of these radiators, these are conventionally infrared radiators, but there is the problem that these also heat the components which surround the optical elements and which, as already mentioned at the beginning, are usually designed from steel to a very great extent. In this way, a very large temperature difference arises between the devices for mounting the optical element and the optical element itself, which leads to a temperature gradient within the optical element.
Therefore, in the area in which the optical element rests on the mountings, a very large point input of heat occurs, since here the mountings, which generally absorb the heat better than the optical element, introduce a very large amount of thermal energy into the optical element, said energy being transported away only inadequately by the latter, since crystalline materials of the type mentioned at the beginning are generally poorer heat conductors than steel, and it being possible for said energy to be distributed in the element. A very high temperature gradient is therefore established in the optical element itself as well, which leads to high thermal stresses in the optical element. This increase in the thermal stress normally takes place in an area in which, as a result of the fact that the optical element is supported on the devices in order to mount it, very high inputs of stress in any case act on the optical element, because of the forces of gravity. It is therefore very easy for a critical shear stress to be exceeded in these areas of the optical element, as a result of the addition of gravitational stress and thermally induced stress. In the case of crystalline materials, this can lead to an offset in the lattice planes or the like, which makes the optical elements unusable for further applications in the area of high-power optics, since plastic deformation occurs.
It is therefore the object of the invention to provide a device which is used to hold an optical element, in particular one made of a crystalline material, during coating of the optical element, in particular by means of vapor deposition of at least one functional layer in a vacuum coating plant, and which avoids the input of thermal loadings into the optical element.
According to the invention, this object is achieved by the features recited in claim 1.
The fact that an intermediate element is used, which has a lower heat absorption than the device for mounting the optical element, means that the thermal energy present in the mountings does not reach the optical element to the full extent. The input of thermal energy into these areas, which are in any case very critical, of the optical element in the area in which it is supported is avoided, and the addition of thermally induced stresses and stresses in the crystal of the optical element, introduced by gravitation, therefore generally remains under a critical shear stress which could effect plastic deformation of the optical element.
In a particularly beneficial refinement of the invention, the intermediate element is additionally connected to the device for mounting the optical element via supporting elements made of a poorly heat-conducting material.
This provides a further advantage. Here, the input of thermal energy from the mountings to the intermediate element, and therefore also to the optical element, can be virtually completely prevented, since the corresponding supporting elements, which can, for example, be designed as small spheres made of ceramic or the like, virtually do not pass on the heat into the intermediate element and therefore into the optical element, in particular also because of their small contact area.
In a further, very beneficial embodiment of the invention, the intermediate element additionally has a coating that reflects the radiation used to heat the optical element.
This coating, which may be composed of gold or the like, virtually completely reflects the radiation used to heat the optical element, so that the intermediate element which, in a particularly beneficial combination of the two embodiments described, is additionally thermally decoupled from the mountings via the supporting elements, is virtually not heated or, in any case, no more than the optical element itself. The input of thermally induced stresses into the optical element, which are caused by local heating and associated high temperature differences between the individual points of the optical element, can therefore be eliminated virtually completely. Coating the optical element with one or more appropriate functional layers is then possible without difficulty, without any impairment of the optical quality of the optical element on account of mutually offset lattice planes or the like, that is to say plastic deformation, having to be feared.
A further advantage is that the intermediate element can be designed in such a way that it has a very good thermal conductivity and, at the same time, a very low heat capacity. It is then possible for the intermediate element virtually always to be at the same temperature as the optical element, since thermal differences are balanced out very quickly as a result of its good thermal conductivity. As a result of the simultaneously very small heat capacity, which can be achieved for example by means of a relatively thin design of the intermediate element, with a very low mass, barely any storage effects, which delay the temperature differences over time, occur in the area of the intermediate element.
In principle, intermediate elements made of corresponding, temperature-resistant plastics would of course also be conceivable here, since these would combine very advantageous properties with regard to heat capacity and heat conduction with very beneficial mechanical properties. However, it should then be ensured that, in the vacuum coating plant, no organic substances can evaporate off from the plastics, which might be deposited on the optical element and could cause impairment of the quality to be achieved of a functional coating on the optical element.